If the format type allows, Surfaces can be exported as either triangulated faces, NURBS or a mix. "As triangles" will break the surface down into small triangular faces that are exported as individual face entries. The tolerance of this tessellation can be controlled on the Preferences dialog. "As NURBS" will export the Surface as an exact mathematical representation of the surface. These formats can support Surfaces in NURBS form: OBJ, IGES, 3DM.
If a format supports both triangles and NURBS the "mix" option will be available, when selected this option will output surfaces as NURBS when possible, and triangles when there is no NURBS representation for the surface (e.g. A triangulated path).
Surfaces exported as NURBS are not textured. Surfaces and Cylinders need to be in triangle form to support photo-textures.
When surfaces are exported as NURBS any defined Trim (outer or hole) will be exported in the formats that support them. In IGES, Trims are exported as type 142/144.
When Surfaces are exported as triangles, each surface is exported as one "mesh" of triangles in the formats that support that form (3DM/Rhino).